This invention relates to a method for tuning a ceramic bandpass filter and, more particularly, to such method that employs selective laser sintering of a metallic paste to adjust a metallized element of the filter to obtain a desired bandpass characteristic.
In a radio receiver or the like, a ceramic bandpass filter is utilized to selectively pass a radio frequency signal having a frequency within a predetermined band referred to as the passband, and to shunt unwanted signals having frequencies outside the passband. A typical filter comprises a ceramic block including one or more elongated cavities extending from a top surface. Metal plates are applied to discrete regions of the block surface to form the necessary electrically conductive elements. These elements include an input pad, an output pad and a ground plate. The cavity is also coated with a metal plate to form a resonator. The resonator includes a resonator pad applied to the top surface of the block and connected to the plate within the cavity. In operation, the resonator is capacitively coupled to the ground plate to shunt the unwanted signals.
The performance characteristics of the filter is determined by the design of the ceramic block and the metal plated elements. For example, the boundaries of the passband are related to the size and shape of the resonator pad. Moreover, the properties are sensitive to slight variations in the dimensions of the blocks and the plates, even within the tolerances normally achievable in mass production. Thus, in mass production, each filter is individually tuned to adjust the properties of the passband. In common practice, tuning is accomplished by forming oversized plates and removing surplus metal while monitoring the output from a test signal applied to the filter until the desired passband characteristics are obtained. The excess metal is removed manually by an operator using a hand-held grinding tool. Because manual grinding is tedious and requires a high level of skill, such tuning is labor intensive and costly. The removal of too much metal by overgrinding results in properties adjusted beyond desired tolerances and thus produces a defective filter that cannot be readily repaired except typically by stripping and replating. Furthermore, by its nature, the grinding operation does not lend itself to automation and is constrained by the limits of operator dexterity in the precision with which filter characteristics may be tuned. For all these reasons, there remains a need for a method for tuning a ceramic bandpass filter that does not rely upon manual grinding of oversized pads to adjust the desired properties of the passband.